1. Field of the Invention
This invention relates to a magnetoinductive flowmeter, encompassing a measuring tube for an electrically conductive medium flowing through it, as well as a magnet with two field coils, which field coils are connected in series for generating an essentially homogeneous magnetic field and, with the aid of a four-switch bridge circuit, are fed a periodically alternating field current equidirectionally flowing through the two field coils by virtue of the alternating opening and closing of two switch pairs, reversing the polarity of the field current. The invention also relates to a method for operating a magnetoinductive flowmeter, said flowmeter incorporating a measuring tube through which flows an electrically conductive medium, two measuring electrodes as well as a magnet with two field coils that are positioned parallel to each other on two opposite sides of the measuring tube and are thus capable of generating a magnetic field that extends in an essentially perpendicular plane relative to the direction of flow, while the measuring electrodes are positioned in a manner whereby their connecting line extends along an essentially perpendicular vector relative to the direction of flow and to the magnetic field and through the cross-sectional center of the measuring tube. In normal flow-measuring operation, the two field coils work in a way whereby an essentially homogeneous magnetic field is generated, permeating the medium as it flows through the measuring tube, its flow rate being determined by means of a voltage collected at one or both of the two measuring electrodes or between the two measuring electrodes under comparison with a reference potential.
2. The Prior Art
Magnetoinductive flowmeters and methods for operating magnetoinductive flowmeters of the type referred to above have been well-known for some time and are used in numerous different fields of application. The underlying concept of a magnetoinductive flowmeter for measuring the flow rate of a medium goes all the way back to Faraday who in 1832 suggested employing the principle of electrodynamic induction for flow-rate measurements.
According to Faraday's law of induction, a flowing medium that contains charge carriers and travels through a magnetic field will develop an electric field intensity perpendicular to the direction of flow and to the magnetic field. A magnetoinductive flowmeter utilizes Faraday's law of induction in that a magnet, generally consisting of two magnetic poles, each with a field coil, generates in the measuring tube a magnetic field perpendicular to the direction of flow. Within that magnetic field, each volume element of the flowing medium that contains a particular number of charge carriers contributes its intrinsically engendered field intensity to a voltage potential that can be collected at the measuring electrodes.
In conventional magnetoinductive flowmeters the measuring electrodes are designed for either direct-electrical or capacitive coupling with the flowing medium. Another salient characteristic of magnetoinductive flowmeters is the proportionality between the measured voltage and the flow rate of the medium averaged across the diameter of the measuring tube, i.e. between the measured voltage and the volumetric flow.
In the actual flow-measuring operation of a magnetoinductive flowmeter the general practice is to periodically alternate the magnetic field. Prior art has developed various approaches to that effect. For example, magnetoinductive flow measurements can be performed using an alternating field, in which case the field coils of the magnet are typically fed alternating current straight from a 50 Hz sinusoidal AC line system. However, transformation interference potentials and line noise can easily distort the flow-generated voltage between the measuring electrodes.
In more recent times, magnetoinductive flowmeters have predominantly employed a switched continuous field. A switched continuous field is obtained by feeding to the field coils of the magnet a current with a time-based, essentially square-wave pattern with periodically alternating polarity reversal. Another possibility is the use of a pulsating constant field which is obtained by only periodically supplying the field coils of the magnet with a sequentially timed square-wave current of always the same polarity. Still, the preferred method involves the periodic polarity reversal of the field current whereby a periodically alternating magnetic field is generated, in view of the fact that this polarity reversal of the magnetic field suppresses interferences such as electrochemical noise.
But then there are other disturbance variables that can affect the magnetoinductive flow measurement. Specifically, the flow measurement may be corrupted when the measuring tube is not completely filled, when the flow profile is inhomogeneous, when there are flaws in the field coils or in the circuitry of the magnet, or when magnetized deposits have accumulated, for instance, on the bottom of the measuring tube.